Efficient second order nonlinear optical (NLO) processes can usually be achieved only in materials having both a large second order nonlinear hyperpolarizability tensor, .beta., at a molecular level, combined with a noncentrosymmetric bulk structure. Studies of NLO properties in organic crystals and powders were initiated almost simultaneously with that of inorganic crystals more than two decades ago. Certain organic crystals satisfy both of the above criteria and show bulk second order nonlinear properties considerably larger than that of traditional inorganic materials, like potassium dihydrogenphosphate (KDP) or lithium niobate (LiNbO.sub.3). However, they do not adequately meet the required standards for practical use in optical devices mainly owing to the great difficulty of growing crystals of sufficient size and quality. Systematic studies of organic materials and comprehensive theoretical explanations of their nonlinear behavior have been emphasized in a number of monographs, for example, in Nonlinear Optical Properties of Organic Materials and Devices, Williams (Ed.), American Chemical Society, Washington, D.C. (1983).
Oriented polymer systems provide an attractive alternative because of their low cost, wide variety and ease of processing. Polymer systems are conveniently prepared either by incorporating the NLO molecules into a suitable polymer matrix or by attaching the NLO molecules covalently to a polymer backbone or side chains. They will be referred hereafter as the `doped` and the `functionalized` polymers or systems, respectively. In both of these systems, the nonlinear optical response arises from the NLO moieties. The `moieties` herein referred to are the organic NLO molecules dispersed in a polymer matrix or the NLO units that are attached to the polymer chains. Background information relating to the principals of nonlinear optical polymers, is contained in Nonlinear Optical and Electroactive Polymers, Prasad and Ulrich, (Eds.) Plenum Press, (1988).
Organic NLO molecules when doped in or functionalized to a polymer matrix, are in an isotropic centrosymmetric organization to begin with. It is essential to establish that the NLO moieties pack the bulk in a noncentrosymmetric fashion for the applications discussed earlier. This is effectively done by using an external electric field, e.g., corona poling, parallel plate poling or integrated electrode poling. The polymer system, after being subjected to an electric field, is referred to as a `poled` polymer. A number of poled polymeric systems which show large nonlinear activity have been reported in recent literature references. Mohlmann et al., SPIE, 1147:245 (1989). However, the decay in nonlinear activities with time handicaps the realization of practical NLO polymers. This is due to the deorientation of the NLO moieties when the electric field is withdrawn.
Polymer systems having inter- and intramolecular crosslinking, have been developed to overcome the relaxation problem associated with the doped and functionalized polymers. Reck et al., SPIE, 1147:74 (1989); Eich et al., J. Appl. Phys., 66(7):3241 (1989). In this system, polymers which exhibit second order nonlinear optical properties are disclosed which are formed by forming a network polymer from monomers during exposure to an electric field. However, network polymerization can substantially interfere with poling of nonlinear optical components, thereby significantly diminishing the optical quality of the resulting network polymer.
Co-pending U.S. Pat. No. 5,112,881, issued to B. Mandal et al. discloses an alternative approach for obtaining crosslinked, second order NLO polymers. Inter- and intramolecular photochemical reactions are used to crosslink the polymer matrix. In this system, a polymer and NLO molecules bearing similar photocrosslinkable compound are processed like a doped polymer. The system can be poled and photocrosslinked in the poled state to yield a material with stable optical nonlinearity.